Nano-crystalline SnO2 particles have been synthesized by sol–gel process using a simple starting hydro-alcoholic solution consisting of SnCl4, 5H2O and citric acid as complexing and ethylene glycol as polymerization agents. The structural properties of the prepared tin oxide nano-powders annealed at different temperatures (300–700 1C) have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. The XRD patterns show SnO2-cassiterite phase in the nano-powders, and size of crystals increases by increasing the annealing temperatures. The TEM images show nano-particles as clusters with size in the range of 5–25 nm. Electron diffraction pattern of nano-powders annealed at different temperatures shows a homogeneous distribution of spherical particles due to the effect of ethylene glycol as polymerizing agent in sol–gel process. The optical direct band gap values of SnO2 nano-particles were calculated to be about 4.05–4.11 eV in the temperature range 300–700 1C by optical absorption measurements. These values exibit nearly a 0.5 eV blue shift from that of bulk SnO2 (3.6 eV), which is related to size decrease of the particles and reaching to the quantum confinement limit of nano-particles.

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